Dio Brando from Jojo. Can't beat the classics. His actions piss me off but I can't help but respect him because he's just so cool.

The worst antagonist would be Sugou from SAO. The show constantly keeps trying to drive in how evil the guy is and make you hate his guts but they fail miserably. I mean HE LITERALLY DRANK ASUNA'S TEARS. What sort of self respecting writer would create a character like that and expect you to take him seriously?

Takasugi and Kamui from Gintama, I always loved how opposite they are from Gintoki and Kagura.

Otherwise, I'd have to agree with the majority and say Hisoka, he is awesome. Honorable mention to Judar (or Judal, however you want to pronounce it) from Magi. I haven't read the manga yet, so I don't really know if he's a true antagonist, but from watching the anime I'm considering him one as of now til something changes. Zancrow from Fairy Tail is another one of my favorites.